In this program a 100 KeV scanning transmission electron microscope (STEM) with resolution better than 2.5 A and high information collecting efficiency will be developed. Thi STEM will be used to image molecules and macromolecules of biological importance with minimum possible degradation by radiation damage. Such information will supplement structural data obtained by x-ray and neutron diffraction. The STEM facility will be jointly used by the Brookhaven scientific staff and by academic scientists. The ability of the STEM to image unstained molecules and to localize single heavy atoms within an organic matrix will be used to locate heavy atom binding sites of biological molecules. The quantitative relationship between electron scattering properties and specimen composition will be used to relate the detected scattering to molecular weight of the imaged molecule. Initial studies will be on nucleic acids (DNA), virus particles, enzyme complexes and structural molecules such as the components of muscle and cell membrane. Studies of thicker specimens such as chromosomes are also planned.